XF 1.2 Get full link of attachment in template

Currently, I'm adding the first attachment into og:image tag and I found this code in some tutorial

<xen:foreach loop="$firstPost.attachments" value="$attachment" i="$i" count="$count">
    <xen:if is="{$i} == 1 AND {$attachment.thumbnailUrl}">
      <xen:set var="$ogThumb">{xen:helper fullurl, $attachment.thumbnailUrl, 1}</xen:set>
This code run well. However, $attachment.thumbnailUrl return the url to thumnail image while I want to get the url to the full image of attachment to make it bigger. Any suggestion for this?

Thank you in advance.